This invention relates to preparation of catalyst employed in a process for hydrodesulfurization of cracked naphtha suitable for gasoline. More particularly, this invention relates to selective desulfurization of cracked naphtha using a catalyst comprising aluminum oxide and cobalt-molybdenum impregnated magnesium oxide.
One of the blending components to be used in a refinery gasoline pool is cracked naphtha containing both sulfur and olefins. The sulfur which is typically present in amounts of about 0.3 wt.% or larger is both a potential air pollutant and a poison to the catalysts used in certain automobile catalytic mufflers. On the other hand, the olefins, which are typically present in an amount of about 30 wt.% or larger, have octane numbers that are higher than those of the corresponding saturates.
Today, sulfur dioxide that is generated by the burning of high sulfur fuels has been identified as one of the chief air pollutants. Hydrodesulfurization is an important method for producing fuels with relatively low sulfur concentrations; however, if the cracked naphtha is to be desulfurized without eliminating or seriously reducing the amount of olefins that are present therein, the desulfurization process must be very selective, i.e., capable of removing substantially all of the sulfur with minimal saturation of olefins (and minimal octane loss). Currently, several desulfurization catalysts that are used in the petroleum refining industry. Such desulfurization catalysts include cobalt and molybdenum and their compounds on a suitable support, nickel and tungsten and compounds thereof on a suitable support, and nickel and molybdenum and compounds thereof on a suitable support. The support, in general, is a non-acidic or weakly-acidic catalytically active alumina.
Bertolacini et al., in copending U.S. Pat. application Ser. No. 820,376, filed July 29, 1977, which is incorporated herein by reference, disclose a process for hydrodesulfurization of cracked naphtha with a catalyst comprising Group VIB and Group VIII metals deposed on a support comprising at least 70% by weight magnesium oxide and exemplifies mixtures with alumina.
While these magnesium oxide supported catalysts are excellent hydrodesulfurization catalysts the magnesia based catalysts can be relatively soft, and can have a relatively low crushing strength with abrasion losses which are commercial disadvantages for both preparing and using the catalyst in refinery operations. Bertolacini et al. disclose that alumina can be incorporated into the support by blending magnesium oxide powder with aqueous solutions of the hydrogenation metals and then sol alumina, with the resultant blend dried, ground to finely divided material, pelleted to an appropriate size, and calcined. Bertolacini et al. also teach, however, that the catalysts incorporating alumina prepared from the sol alumina blend have reduced selectivity in comparison to those catalysts prepared with an entirely magnesium oxide support.
Yu et al. in copending U.S. Pat. application Ser. No. 892,389, filed Mar. 31, 1978, which is incorporated herein by reference, disclose that the selectivity of the Bertolacini et al, catalysts can be improved by using relatively low levels of the hydrogenation metals. Yu et al. further disclose that crushing strength of the magnesia catalyst can be improved by employing molybdenum sulfide as the pilling agent or pelleting lubricant.
Wight, in U.S. Pat. No. 3,923,640 (1975), discloses preparation of hydrocracking catalyst prepared from zeolite cracking base of the low-sodium Y-type, wherein the zeolite component can be intimately admixed with a finely divided, hydrous, refractory metal oxide such as alumina; the metal oxide can also be combined with the zeolite as a hydrosol or gel, as an anhydrous activated gel, as a spray dried powder, a calcined powder, or the metal oxide precursor can be precipitated to form a gel in the presence of the zeolite. The Wight patent discloses that when mulling the zeolite with the finely divided form of the metal oxide, minor amounts of water, with or without an acid peptizing agent, are usually added to facilitate admixture.
Lefrancois, in U.S. Pat. No. 3,269,938 (1966), discloses preparation of silica-magnesia supported catalyst comprising molybdenum and nickel wherein the silica-magnesia composite is prepared prior to incorporation of the molybdenum and nickel precursors by methods of coprecipitation or by adding magnesia as a slurry in water to an acidic silica hydrosol and allowing the resulting sol to set to a hydrogel, followed by drying and calcining the hydrogel.
The general object of this invention is to provide a catalyst having improved crush strength and improved selectivity in the desulfurization of cracked naphtha in order to minimize octane loss in the product by reducing olefin saturation in the hydroprocessing. Other objects appear hereinafter.
I have found that the objects of this invention can be attained with a catalyst prepared by dry blending particulate alumina with a dried composite of magnesia impregnated with hydrogenation metals of Group VIB and Group VIII of the Periodic Table of Elements, according to the following method:
(1) forming an aqueous composition comprising dissolved Group VIB and Group VIII metal compounds and suspended magnesia; PA1 (2) drying the composition of step 1; PA1 (3) blending compositions comprising the product of step 2 and particulate alumina; PA1 (4) forming the product of step 3 into pellets; and PA1 (5) calcining said pellets.
I have unexpectedly found that in contrast to the alumina containing catalysts disclosed in the applications of Bertolacini et al. and Yu et al., which are prepared from alumina sol, improved selectivity can be achieved by incorporating alumina in the catalyst by the method of this invention, that is generally when particulate alumina is dry-blended prior to pilling or forming by adding typically powdered alumina as an admixture with the impregnated magnesia prior to such pelleting. I have further found that magnesia based catalyst incorporating alumina by the method of this invention produces improved selectivity over even the low-metals catalyst containing entirely magnesia support, disclosed by Yu et al.
Broadly, the catalyst of this invention can be prepared by impregnating commercially available magnesia, for example the magnesium oxide powder available from Mallinckrodt Chemical Company and from Basic Chemical Company, with heat-decomposable compounds of the Group VIB and Group VIII hydrogenation metals from generally, either an aqueous solution containing both metal compounds or a solution containing one of the metal compounds followed by a solution containing the other metal compound. The resulting composite can be stirred or mixed to a "paste" and then dried, generally in air, either in the mixing vessel or in separate equipment at a temperature of about 250.degree. F.-450.degree. F. for a period of 1 to 20 hours. As used herein, the term "paste" refers to the impregnated magnesia prior to drying, whether the material has paste-like, merely coagulated, or any other consistency. While not essential, the dried paste can be ground. The dried paste of impregnated magnesia can then be dry-blended with particulate alumina, commercially available generally as granules or as a powder whether in hydrate form such as the alpha monohydrate, boehmite, or in calcined form such as gamma alumina; other aluminas such as eta alumina and its hydrate precursors can also be similarly dry blended. While the major component of the catalyst support is magnesium oxide, sufficient particulate alumina can be blended to provide a calcined catalyst comprising about 5 to about 50 wt.% aluminum oxide, preferably about 5-30 wt.%; catalyst comprising about 8-12 wt.% aluminum oxide is particularly selective in hydrodesulfurization of cracked naphtha as later described.
The hydrogenation component of the catalyst comprises a Group VIB metal and a Group VIII metal, with the Group VIB metal present in an amount of about 4 wt.% to about 20 wt.%, preferably about 4-6 wt.%, and the Group VIII metal present in an amount of about 0.5 wt.% to about 10 wt.%, preferably about 0.5-2 wt.%, each amount being based on the total weight of the catalyst and being calculated as the oxide of the respective metal. The catalyst comprises at least one Group VIB metal selected from chromium, molybdenum and tungsten and at least one Group VIII metal selected from iron, cobalt, nickel, ruthenium, rhodium, platinum, palladium, osmium and iridium. The preferred Group VIB metal is molybdenum and the preferred Group VIII metal is cobalt. Typical water soluble salts such as cobalt nitrate, cobalt acetate, cobalt formate, and ammonium heptamolybdate can be employed to impregnate the magnesia. Under desulfurization conditions, these metals are present in the catalyst in at least one form selected from the elements, their oxides and their sulfides.
The dry blended alumina and impregnated magnesia can be formed into pellets by pilling or tableting with a conventional pilling machine or pellet mill such as those manufactured by Colton and Manistee. Pilling produces strong, uniform pellets even with the difficulty formed magnesia-based feed when powdered lubricant, such as MoS.sub.2, graphite, or a vegetable-based powder is added to the impregnated magnesia with the particulate alumina in the dry blending operation. MoS.sub.2 is generally added at a level of about 1 wt.% of the dry blended mixture; graphite lubricant can be added at a level of about 2 wt.% of the dry blended mixture; and powdered, vegetable-based lubricants can be added at a level of about 5 wt.% of the dry blended mixture. Best results have been obtained employing molybdenum sulfide. The pill size and properties can be controlled by the amount of fill in the dies and the machine's compression setting. The green pellet crush strength is the strength in psig before calcination.
Forming magnesia-based catalyst pellets using conventional extrusion equipment can require that water be added to the dry blended mixture of alumina and impregnated magnesia to produce an extrudable plastic; alternatively the paste of metal compounds and magnesia can be subjected to relatively limited drying before blending with the particulate alumina in order to obtain an extrudable plastic composite.
Calcination of the catalyst pellets can be carried out at a temperature as low as about 450.degree. F. for a period of about 1-2 hours followed by a temperature of about 700.degree. F. to about 1100.degree. F., preferably, about 800.degree. F. to about 1000.degree. F., for a period of about 1.5-10 hours.